Optical sighting and observation devices used in military applications, such as a tank gun sight, frequently possess "cat's eye" reflection or "retro-reflection" characteristics. That is, light incident on the device is reflected from one or more lens, mirror, prism or other optical surfaces and redirected out of the device in exactly the direction from which it came. Thus, particularly in a night environment, the sighting device may effectively act as a beacon, transmitting light back to an enemy illuminating the area with light or other illumination. The adverse consequences of revealing the operator's position in this manner are readily apparent. Hence, a formidable objective in designing and constructing military sighting devices is to eliminate retro-reflection characteristics.
Although all optical surfaces within a given optical system may contribute to retro-reflection, some optical elements contribute more prominently to retro-reflection than other optical elements. The optical element addressed herein is the optical reticle. The optical reticle is the cross-hair optical element provided in the focal plane as a sighting reference. The optical reticle usually consists of a plano-plano polished glass element with fine lines or grooves etched therein and filled with a diffusely reflecting solid such as titanium dioxide. When the reticle is illuminated from the side, preferably with an appropriate red illumination when used at night, the solid material stands out and serves as a highly visible reference for the user. Unfortunately, however, the reticle is a large contributor to the retro-reflection and, hence, the optical "signature" of any given optical sighting device. This is due to the fact that light rays collected by the sighting device and incident upon the polished surface of the reticle are redirected back through the optical system toward the source, i.e. the enemy, without any reduction in intensity due to angular dispersion effects. A simplified optical diagram of this effect is shown in FIG. 1.
Several attempts have heretofore been made by the inventor and others to reduce the undesirable retro-reflection characteristics of the optical reticle.
One such attempt, commonly known as the sandwich cemented reticle, involves cementing a glass cover plate to the reticle. Based upon known optical principles: ##EQU1## where R=reflection,
N.sub.m =refractive index of the medium, i.e. glass plate adjacent to the reticle, and PA1 N.sub.R =refractive index of the reticle substrate.
By way of example, for a traditional reticle having no cover plate N.sub.m =1 (i.e. the index of refraction of air) and N.sub.R is approximately 1.5 (i.e. the index of refraction of glass). Based upon Equation 1 above, the amount of reflected light is approximately 0.04 or 4 percent reflectance.
For a sandwich cemented reticle the reflectance in accordance with Equation 1 will theoretically approach zero as the indices of refraction N.sub.m and N.sub.R are made equal. However, sandwich cemented reticles actually involve three optical media: the reticle substrate, the adjacent cover plate, and an optical cement between two reticle substrate and cover plate. Thus, the indices of refraction for all three optical media must be identical in order for the reflectance R to approach zero. Otherwise, the sum of the reflections at the cover plate to cement interface and the cement to reticle interface will be appreciable. The solution to this matching problem has proven elusive. Indeed, aging, humidity, temperature, infra-red exposure and numerous other potential problems may affect optical stability and over time and destroy what had been believed to be ideal optical matching conditions. In particular, such effects over time can alter the properties, including index of refraction, of optical cements. Furthermore, it is difficult to obtain appropriate optical quality glass and cements having identical indices of refraction. Some optical cements are specially prepared to match reticle substrate and cover plate refractive indices, but such cements do not provide sufficient refractive index control to yield the desired degree cf reflection suppression.
Thus, attempted solutions simply do not provide sufficient, reliable reflection suppression. Moreover, even when occasional satisfactory results have been obtained, such results have not proven to be repeatable on a regular basis. Therefore, occasionally successful test results have not necessarily indicated a basis for a marketable production item having the desired reflection suppression characteristics. Such occasionally successful results certainly cannot form the basis for a repeatable production item to satisfy traditionally strict military specifications.
Other solutions have been attempted, such as projected reticles which are viewed indirectly rather than etched and filled reticles which are viewed directly. However, these other solutions suffer from other drawbacks, such as not being viewable against bright backgrounds, and are outside the scope of the present invention.
Therefore, it is one object of the present invention to provide an optical reticle having a high degree of reflection suppression.
It is a further object of the present invention to provide an optical reticle having a high degree of reflection suppression which can be produced repeatably.
It is a further object of the present invention to provide an optical reticle of the etched and filled type having a high degree of reflection suppression.
These and other highly desirable and unusual results are accomplished by the present invention in a compact structure which can achieve a high degree of reflection suppression on a repeatable basis.
Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.